1. A Novel TDS-FDMA Scheme for Multi-user Uplink Scenarios
Linglong Dai, Zhaocheng Wang, Jun Wang, Zhixing Yang
Electronic Engineering, Tsinghua University, Beijing, China
Background and Motivation
B. Joint Channel Estimation
The actual received m-sequence at the TDS-FDMA receiver intrinsically inherits the cyclicity property due to the cyclic prefix in the preamble and takes the form
Using a local m-sequence to do circular correlation with the received m-sequence
The uer-specific CIR can be directly selected from
OFDMA and TDS-OFDM
OFDMA was widely used in cable TV (CATV), the uplink of the interaction channel for digital terrestrial television (DVB-RCT), downlink of LET, uplink and downlink in IEEE e [1].
Time domain synchronous OFDM (TDS-OFDM) is the essential technology of the Chinese national digital television terrestrial broadcasting (DTTB) standard [2].
TDS-OFDM adopts the pseudo-random noise (PN) sequence as the guard interval (GI), which is also used for synchronization and channel estimation (CE), thus spectral efficiency is improved.
The cyclicity of the received IDFT block is destroyed due to the inter-symbol interference (ISI) between the PN sequence and the IDFT block, and some iterative interference cancellation algorithm have been investigated, but the complexity is very high [3-5].
Main Issue When TDS-OFDM Is Used in The Uplink Multi-User Scenarios
Until now, TDS-OFDM has been used only in the downlink broadcasting transmission.
Superimposed ISI caused by multiple users in the uplink is very difficult to be removed, and the conventional iterative methods [3-5] can not be used any more, where CE and interference cancellation are mutually conditional.
C. Spectral Efficiency
Both the frequency-domain pilots and the time-domain guard interval (CP) result in the decrease of the spectral efficiency.
For DVB-RCT,
where and are the number of useful data symbols and the number of pilot symbols in three types of burst structure (BS) defined by DVB-RCT, i.e., BS1, BS2 and BS3, respectively
For the proposed TDS-FDMA,
Table I compares the spectral efficiency between DVB-RCT and TDS-FDMA
The increase of the spectral efficiency by about 5%∼9% can be achieved by the TDS-FDMA system.
Contributions
Based on the TDS-OFDM and FDMA principles, a novel uplink multiple access scheme called time domain synchronous FDMA (TDS-FDMA) is proposed in this paper.
The specially designed frame structure makes the CE and interference cancellation in the proposed TDS-FDMA system no longer mutually conditional, so the conventional iterative methods can be avoided, leading to the low complexity of the joint cyclictiy reconstruction of the received IDFT blocks.
The preamble in the frame structure enables joint channel estimation for all users to be realized by an one-step circular correlation.
TABLE I. Spectral Efficiency of DVR-RCT and TDS-FDMA
Proposed TDS-FDMA Transmission Scheme
D. Computational Complexity
Compared with CP-OFDM, the higher complexity of the TDS-OFDM system is primarily caused by the iterative interference cancellation for cyclicity reconstruction and channel estimation.
Table II compares the computational complexity of the single-user TDS-OFDM receiver and the proposed multi-user TDS-FDMA receiver ( , J is the iteration number)
The computational complexity of the proposed multi-user TDS-FDMA receiver is only 6.1% of the traditional single-user TDS-OFDM receiver adopting Wang’s method [9], and 6.0% of the Tang’s method [10], 35.1% of the Yang’s method [11], respectively.
Fig. 1 Frame Structure of the TDS-FDMA System.
TABLE II. COMPLEXITY COMPARISON BETWEEN TDS-OFDM AND TDS-FDMA.
Fig. 1 shows the frame structure for the mth user of the proposed TDS-FDMA system.
The basic block transmission unit is the superframe, which is composed of one preamble and L subframes.
The preamble consists of three parts: the user-specific Np-point m-sequence , the K-point prefix and the K-point postfix
For the multiple users locating at different places, the user-specific m-sequences between neighboring users hold a constant circular shift :
The Nf-point subframe consists of the N-point IDFT block and the K-point postfix , the postfix is not relative to the IDFT block in every subframe, but identical with the prefix and postfix in the preamble
The IDFT block could be either the OFDMA type of multi-carrier (MC) signal or the single carrier FDMA (SC-FDMA) type of single-carrier (SC) signal [12] .
The proposed frame structure provides a uniform frame structure both for the MC and SC multiple access uplink scenarios.
Simulation Results
The multi-user TDS-FDMA system achieved a superior BER performance over the TDS-OFDM System.
In the QPSK case, an SNR improvement of about 1.8 dB and 5 dB when was achieved by the multi-user when fd = 5 Hz and fd = 20 Hz, respectively. However, the SNR gain became negligible for the high Doppler spread of fd = 50 Hz.
The improved BER performance is due to more accurate CE, and joint cyclicity reconstruction does not need any CSI at all (Thus it avoids the accumulated errors due to the imperfect CE and residual ISI during the iteration process).
When the channel is varying too fast, the preamble based CSI updating speed becomes relatively low which counteracts the performance gain explained above. This problem can be solved by reducing L..
Proposed TDS-FDMA Receiver Design
A. Joint Cyclicity Reconstruction
The one-step add-subtraction operation between the received IDFT block, the postfix in the subframe and the postfix in the preamble would achieve the joint cyclicity reconstruction of the received IDFT block.
Fig. 3. BER performance comparison between
TDS-FDMA and TDS-OFDM over Vehicular-A fading channel with the QPSK modulation scheme.
Fig. 4. BER performance comparison between
TDS-FDMA and TDS-OFDM over Vehicular-A fading channel with the QPSK modulation scheme.
Conclusions
The major difficulty of eliminating the superposed interferences from multiple users when TDS-OFDM is extended to multi-user uplink scenarios is resolved by the proposed TD-SFDMA scheme in this paper.
TD-SFDMA can achieve higher spectral efficiency by about 5%∼9% than typical OFDMA like DVB-RCT
A novel frame structure is proposed to separate channel estimation and cyclcitiy reconstruction, and the corresponding receiver algorithms can be realized with lower complexity for both the MC and SC uplink transmission
TDS-FDMA scheme could be adopted as a potential uplink solution to the Chinese next generation broadcasting standard.
Fig. 2 Joint cyclictiy reconstruction of the received IDFT block for all M users in the TDS-FDMA system.